This invention relates generally to sealed systems and, in particular, to apparatus and methods for equalizing the pressure within sealed systems.
A non-pressurized, fixed-volume sealed system or enclosure may be hermetically sealed to isolate the enclosed air space from contaminants, such as water vapor, in the surrounding ambient atmosphere. Certain of these sealed systems are exposed to temperature variations arising due to the transfer of heat to and from housed components or an external source. The temperature variations modulate the pressure of the air, or other gas, filling the hermetically-sealed enclosure. In particular, sealed systems exposed to the daily heating and cooling of an outdoor environment experience cyclic pressurization and depressurization due to volume changes in the enclosed gas. Specifically, absorption of solar radiation by the material forming the sealed system transfers significant amounts of heat energy to the gas inside the sealed system, which can attain a temperature significantly higher than the ambient temperature surrounding the sealed system. Generally, the pressure variation within a sealed system at most practical temperatures of interest is about 1.5 p.s.i. per each 60xc2x0 F. temperature increment.
Communications systems incorporate non-pressurized, sealed systems, such as waveguides and transmission lines, that enclose fixed volume air spaces. Such sealed systems typically feature RF-transmissive windows susceptible to mechanical damage or failure if the enclosed gas, when heated, exerts an excessive positive or outwardly-directed pressure. Alternatively, the external air pressure may exert an inwardly-directed pressure when the enclosed gas is cooled that is sufficient to cause the RF-transmissive window to be damaged or to implode. In addition, such sealed systems may incorporate multiple sections united by sealed junctions relying on conventional sealing members such as compressed elastomeric o-rings or gaskets. During a heating period, the enclosed gas becomes pressurized and exerts an outwardly-directed force at each sealed junction. The outwardly-directed force can compromise the ability of the sealing member to provide an effective seal so that gas may breach the sealed junction and escape from the sealed system to compensate for the increased pressure. As the sealed system cools after the heating period, the decreasing pressure of the gas can aspirate air from the ambient atmosphere past the seals and into the sealed system. The aspirated ambient air entering the sealed system can be laden with water vapor or other contaminants.
Other types of sealed systems are enclosures incorporate an access opening covered by a reclosable closure. The access opening is dimensioned to permit manual entry, when the closure is removed, into the interior space of the hermetically-sealed enclosure. A sealing member is typically compressively captured between the outer periphery of the closure and the inner periphery of the access opening to provide an air-impermeable seal. During a heating period, the closure experiences an outwardly-directed motive force proportional to the exposed area of the closure as the pressure inside the hermetically-sealed enclosure increases. The outwardly-directed force reduces the effectiveness of the seal so that the enclosed air escapes past the sealing member to compensate for the increased pressure. Closures on larger hermetically-sealed enclosures generally have a larger surface area upon which the outwardly-directed pressure can act and, therefore, will experience greater outward net forces during the heating phases of the cycle so that the detrimental effect of the thermal cycling is amplified. As the hermetically-sealed enclosure cools after the heating period, the decrease in the internal pressure can aspirate air laden with water molecules from the ambient atmosphere past the sealing member and into the enclosure.
The water vapor in the ambient air admitted into the sealed system or enclosure condenses as water on the moisture-sensitive surfaces and any electrical components inside the enclosure, with deleterious effects. In transmission lines and waveguides used in communications systems, condensate causes corrosion and oxidation that increase attenuation and that can permanently or intermittently degrade the system performance. Another effect is that condensate can create a conductive pathway between the inner and outer conductors of transmission lines that can lead to voltage arcing and subsequent failure. For electrical components, the condensate corrodes and oxidizes electrical contacts. Therefore, an important design consideration for sealed systems is to prevent condensation at the lowest potential temperature to which the sealed system is cooled.
Two conventional approaches have been used to provide pressure equalization in sealed systems having non-pressurized air spaces that require a dry environment. One approach is to provide a sidewall of the sealed system with an expandible diaphragm. As the pressure within the sealed system increases and decreases as a function of temperature, the diaphragm expands and contracts to adjust the total volume of the sealed system for maintaining a constant internal pressure. However, when the sealed system is hermetically sealed, the environment inside the system will reflect the atmosphere in which the system was sealed. Unless measures are taken to provide a dry gaseous environment with a suitable dew point, the relatively humid air trapped inside the sealed system during the sealing process will contain significant moisture. The moisture provides a readily available source of condensate. In addition, each time the sealed system is opened in an ambient environment, relatively humid air will fill the system unless suitable precautions are taken when the system is resealed. Moreover, inwardly-directed leakage from the ambient environment due to imperfect sealing can introduce humid air from the ambient environment.
Another approach is to vent the sealed system to the ambient atmosphere through a fluid passageway that includes a static desiccant. The desiccant removes moisture from the ambient air entering the sealed system as the pressure inside drops. Typically, the desired dew point inside the sealed system is less than about xe2x88x9240xc2x0 C. to about xe2x88x9245xc2x0 C. which corresponds to about 0.2% relative humidity. However, desiccants in such pressure equalization apparatus become saturated with moisture and must either be intermittently regenerated or replaced. In addition, if the sealed system is opened and resealed, it takes many air exchanges during heating cycles to effectively lower the humidity back to the desired level.
Often, sealed systems are found in positions that are not readily accessible so that the absorbent or desiccant cannot be easily serviced when saturated. For example, desiccated vents for tower-mounted sealed systems can only be serviced if a technician climbs to the top of the tower or uses a crane or a lift to gain access. In addition, any operating equipment near the sealed system must be idled while the technician services the desiccated vent for safety reasons, which disrupts service and increases maintenance costs.
Therefore, it would be desirable to have an apparatus and associated methods for regulating the internal pressure of a gas within a sealed system while maintaining the gas at a characteristic dew point and that can do so while lengthening the lifetime of the adsorbent dehumidifying the gas to provide the characteristic dew point.